I had both knees replaced at Mayo Phoenix last year 3 months a part. I am a super active 73 year old. I experienced a little bit of the same things you are feeling after the second surgery. Surgery does a number on your body. It is traumatic to your system. I give blood at the Red Cross about every 8 weeks. When I went in about two months after surgery, my hemoglobin was lower than their policy allows for blood draw. I was surprised, but that was the reason for being a bit lethargic, workouts were not as aggressive, I tired more easily, had a bit of low stamina. I was anemic according to the numbers. When I discussed this issue with the fine doctors at Mayo, and after reviewing if I had any other trauma or blood loss, they suggested I CAREFULLY take an iron supplement and that . Too much iron can be dangerous. In Arizona you can schedule your own battery of blood tests without a prescription. I think the hemoglobin test was $25.00. I did it to double check what the Red Cross numbers indicated. It was low: 12.1. It should between 13.8 and I think 17. Even with taking the iron supplement, it two more months to get it to my pre-surgery level of 15.2. Talk to your Doc. This may be what's happening.